The present invention relates to couplings for fluid systems, and, more particularly, to a coupling for connecting to such a system a device, such as a sampler, that is not pre-filled. The invention further relates to an improved sampler which minimizes the amount of included air.
When any device is to be filled with fluid from a closed system, it is difficult to avoid contaminating the device or the system generally with included air. The term "included air" is used here to refer to air which is present within the coupling or its surroundings prior to connection and does not refer to air entrained or dissolved within the fluid. Thus, the term included air embraces all extraneous air that does not come from the system to be sampled; it embraces air from within the internal volume of the coupling and ambient air that becomes included as the coupling is connected.
The need to avoid included air is particularly important when a sampler is to be connected to the system to withdraw a sample of fluid which will later be analyzed to measure its entrained or dissolved air content. Samples are taken for this purpose from the working fluid of aircraft hydraulic systems and the oil in transformers, to name two examples.
There are several known couplings which are used to make connections to fluid systems, exemplary couplings being described in this inventor's previously issued U.S. Pat. Nos. 3,073,342 and 3,778,092. They employ an arrangement of a valve head surrounded by a conical valve seat. Upon engagement with a coupling receiver, the seat moves away from the head, permitting the fluid to flow through an annular opening between the seat and the head, parallel to a valve stem to which it is connected, and out the other end of the coupling. Flow in the opposite direction is also permitted.
These previously known couplings are primarily suitable for use in those situations in which fluid is present on both sides of the coupling, i.e., the device to be connected to the system is pre-filled, but could also be used in non-prefilled situations in which included air is of limited concern. It has been found, however, that the above-mentioned couplings do not perform satisfactorily when connected to a fluid sampler to be used to test a fluid for air content. A large volume of air that surrounds the valve stem, behind the valve seat and head, is drawn into the sampler along with the system fluid, and this included air makes accurate analysis of the system fluid impossible.
Other structurally different fluid couplings are known in which the coupling valve is formed by an inner member and a sleeve that slides axially on the inner member. The inner member has an axial passage that communicates with ports on its cylindrical sides. To open the valve, the sleeve is moved into a position in which the ports are exposed.
Previously known valves of this last-mentioned type have a relatively large internal volume within the ports and axial passage and, therefore, present the same included air problems as do those valves discussed above. A further disadvantage of these valves is that they rely upon O-rings to achieve a seal between the inner member and the sleeve, the forwardmost O-ring being engaged and disengaged as the valve is closed and opened. The member that engages and disengages this O-ring must have a chamfered leading edge if the O-ring seal is not to be worn excessively or otherwise damaged. The presence of this chamfered edge creates a void when the coupling is mated with the coupling receiver and this void is an additional source of included air.
There is at present a need for a fluid coupling that minimizes the amount of included air when a connection is made, particularly when the device to be connected is empty prior to connection. There is also a need for a sampler that is capable of withdrawing a fluid sample while minimizing contamination of the sample by included air. The objective of the present invention is to fill the above needs with an improved coupling that retains the important advantages of previously known fluid couplings, such as fast connection and disconnection, high reliability and freedom from fluid leakage.